Recently, there is known a chip for minimizing standby power consumption. When such a chip for minimizing standby power consumption (hereinafter referred to as “EcoChip”) is mounted on a communication device such as a cell phone, the communication device can always monitor an access point (hereinafter referred to simply as an “AP”) and a radio signal transmitted from other communication device. More specifically, for example, when an EcoChip is used to perform wireless LAN communication between a cell phone and an AP, the power of the WLAN communication module with large power consumption in the cell phone is turned off and the EcoChip with small power consumption can be used to monitor a signal from the AP on a regular basis.
Here, the EcoChip in the cell phone determines whether a standby signal transmitted from an AP or the like is detected or not, only by a receiving intensity (high or low) on a time axis. For this reason, the EcoChip determines the presence or absence of a required detection based on a unique transmission sequence or the periodicity of the signal continued to be transmitted at a constant period.
The EcoChip may use a reference frequency generated by a crystal oscillator in the circuit as an operating clock for operating the EcoChip. Alternatively, an atomic clock can be considered to be used as an operating clock for operating the EcoChip. However, the atomic clock depends on the radio wave receiving circumstance, and thus it is difficult to use the atomic clock in a room or particularly underground.
Here, the precision of the crystal oscillator depends on the temperature (frequency-temperature characteristics). An oscillation frequency of the crystal oscillator may be cause increase and decrease of several 100 ppm depending on the ambient temperature. For this reason, if a crystal oscillator is to be used as the operating clock for operating the EcoChip, the oscillation frequency of the crystal oscillator needs to be corrected on a regular basis or on an event-driven basis.
Although it may be considered that a more high-precision crystal oscillator is installed in the circuit as the operating clock for operating the EcoChip, reserving a new space for the high-precision crystal oscillator is required. Moreover, a method of correcting the oscillation frequency by a feedback circuit or the like increases the power consumption of the EcoChip. Thus, it is difficult to achieve low power consumption by the EcoChip.